1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method and a program that process a color imaging signal obtained by an imaging device in which a plurality of types of imaging elements having different spectral sensitivities are regularly aligned on a single plane.
2. Description of the Related Art
An apparatus that electrically acquires a color image can be roughly divided into an apparatus having a three-chip imaging element configuration that can simultaneously obtain three color (R, G, B) signals at one pixel position and an apparatus having a single-chip imaging element configuration that can obtain one color signal alone in three color signals at each pixel position. Currently commercially available digital cameras generally have the single-chip imaging element configuration.
Although the three-chip imaging element configuration is expensive since its configuration is complicated and the number of its components is large, it can simultaneously obtain three color signals at each pixel position of an acquired image, and hence it generally has a high image quality.
On the other hand, the single-chip imaging element configuration is a simple configuration, but R, G and B filters must be arranged in, e.g., a mosaic pattern which is called a Bayer arrangement shown in FIG. 14 in accordance with each pixel in order to obtain three color pixels.
In such a Bayer arrangement, the three color signals are obtained at each pixel by interpolating missing color signals at respective pixel positions with color signals at peripheral pixel positions.
The easiest method for reducing such a false color is forming on an imaging element an image having characteristics of a lens or an optical low-pass filter reduced to a spatial frequency that can reproduce the sampling interval of the R signal or the B signal in the Bayer arrangement.
Therefore, it is general to design an optical system in such a manner that an image resolution for forming an image on an imaging element has spatial frequency characteristics that do not produce moire due to aliasing distortion with the sampling interval of the G signal.
Various types of methods for using such an optical system and reducing a false color have been conventionally proposed.
For example, JP-A 7-059098 (KOKAI) suggests a method for switching interpolation filters in adapting to the direction dependency of a G signal similarity in a local region of an image, an inter-color-signal similarity of a G signal and an R signal, or an inter-color-signal similarity of the G signal and a B signal.
That is, in JP-A 7-059098 (KOKAI), horizontal and vertical similarities of a total of four upper, lower, left and right G signals around a pixel position of a missing G signal are calculated, and the calculated similarities are compared with a predetermined threshold value to select one from three linear interpolation methods using the G signal.
FIGS. 15A to 15C show this interpolation method. FIG. 15A shows a method for calculating a similarity |G1−G2| of a G signal that is adjacent to a value X of a central G signal in the vertical direction and further executing a linear interpolation arithmetic operation “(G1−G2)/2”. FIG. 15B shows a method for calculating a similarity |G3−G4| of a G signal that is adjacent to a value X of a central G signal in the horizontal direction and further executing a linear interpolation arithmetic operation “(G3+G4)/2”. Furthermore, FIG. 15C shows a method for calculating similarities |G1−G2| and |G3−G4| of a G signal that is adjacent to a value X of a central G signal in the respective vertical and horizontal directions and further executing a linear interpolation arithmetic operation “(G1+G2+G3+G4)/4”.
As described above, the technology disclosed in JP-A 7-059098 (KOKAI) selects one from the three linear interpolation methods.
Such a method has an inconvenience that, when an input single-chip color imaging signal has a band limiting effect reduced due to an optical low-pass filter and others and a phenomenon such as color moire due to frequency aliasing at the time of photoelectric conversion has been already included, jaggy is produced at an oblique edge (JP-A 7-059098 (KOKAI)).